1. Field of the Invention
The present invention relates to a process for producing lube base oils of high viscosity index by hydroisomerization and hydrocracking of paraffinic hydrocarbon streams. More specifically, the present invention relates to a process for hydroisomerizing/hydrocracking which leads to lube base oils of high viscosity index and low pour point by contacting a feed enriched in iso-paraffins with a catalytic bed made up of a hydrodemetallization catalyst (HDM), a hydrotreating catalyst (HDT), a hydroisomerization/hydrocracking catalyst (HDI/HCC) and a hydrofinishing catalyst (HDF) and further separation of the product obtained.
The present invention relates further to a process for producing high quality fuels from the same paraffinic hydrocarbon streams and same catalytic bed, by recycling the 370.degree. C.+ cut to the process feed.
2. Prior Art
Present engines and machinery require that the feed used to produce lube oil be dewaxed to eliminate high molecular weight n-paraffins as well as slightly branched paraffins which increase the pour point as well as the cloud point of the oils. Thus, to obtain sufficiently low pour points, these waxes must be completely or partially eliminated. The removal of the waxy compounds from the lube oils can be made by solvent dewaxing or either by a catalytic process, this latter being the most economical approach, since it works by selectively cracking the longer n-paraffins as well as the mono-methyl-paraffins, to yield lower molecular weight products, the separation of some of these being possible by distillation.
Among the dewaxing catalysts one can cite the zeolitic aluminosilicates, the pore size of which allows the entrance of linear n-paraffins and some slightly branched paraffins (thereafter in the present specification called waxes),while other materials such as more branched paraffins and cycloaliphatic and aromatic compounds are excluded. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23 and ZSM-38 have been described as being adequate for the desired dewaxing. See for example U.S. Pat. Nos. 4,247,388 and 4,659,311. However, those catalysts are not useful for isomerizing highly paraffinic feeds, since the cracked products are of excessively low molecular weight. Thus, feeds of more than 80% by weight of n-paraffins and mono-methyl paraffins require another approach in order to produce lube oil in high yield and of low pour point.
Catalytic hydroisomerization is one of the possible ways of treating feeds of high paraffin content, the catalyst being a zeolite of intermediate pore as disclosed in U.S. Pat. No. 4,734,539 or in U.S. Pat. No. 4,518,485 wherein the dewaxing is effected through a first hydrotreating step in order to remove at least part of the sulfur and nitrogen compounds followed by contact of the feed with beta zeolite wherein the silica/alumina ratio is at least 30:1 and having a hydrogenation/dehydrogenation component, the n-paraffins being converted by isomerization.
U.S. Pat. No. 4,547,283 teaches a catalytic isomerization process from slack waxes (oily waxes produced by solvent dewaxing the bottoms of heavy distillate hydrocracking units) wherein the paraffin which has not been isomerized is recycled after a further solvent dewaxing process.
WO 92/01769 teaches an isomerization process for paraffinic feed aiming at producing a lube oil wherein the catalyst is an inorganic oxide molecular sieve of oval pores 1-D having a secondary axis between 4.2 .ANG. and 4.8 .ANG. and a main axis between 5.4 to 7.0 .ANG., and at least one metal of Group VIII of the Periodic Table. The catalyst comprises a silicoaluminophosphate molecular sieve of intermediate pore such as a ZSM-22, ZSM-23 or ZSM-35 zeolite and at least a Group VIII metal. It is alleged that the lube oil so obtained shows a high level of paraffins which causes a viscosity level which is low for its boiling range. These properties cause less attrition strength, less engine wear and higher fuel efficiency. Cold starts are better. Oil viscosity index (V.I.) is between 125 to 180 while the pour point (P.P.) is between -63.degree. C. to -24.degree. C. Since oil characteristics allow its use free of additives, there are less deposits which are harmful to the engine performance. Due to the high paraffin content of the oil, oxidation resistance is improved.
European EP 321307 uses an approach aiming at protecting the hydroisomerization catalyst through the previous removal of compounds such as sulfur, nitrogen and polynuclear aromatics by contacting with a typical hydrotreating catalyst of small pore diameter such as Co--Mo on alumina.
In the present invention, a hydrodemetallization catalyst is used, aiming at removing polynuclear aromatic compounds as well as asphaltenes, the pore size of which allows the easy access of the more bulky molecules to the active sites.
Besides, the feeds described in the literature are considered as noble ones since they generally stem from a gasoil hydrorefining process or either from aromatic extraction of distillates, these processes yielding paraffinic oils which when submitted to solvent dewaxing yield a noble paraffinic cut called slack wax as well as an oil which should be hydrofinished. The slack wax contains in its composition a high level of n-paraffins of high melting point which after separation and hydrofinishing are of high commercial worth.
Brazilian Application BR PI no 9003449 of the Applicant (now corresponding to U.S. Pat. No. 5,205,923 and hereby fully incorporated as reference) teaches that the withdrawal of color-forming compounds from macro- and microcrystalline waxes is effected by contacting same with a mixed bed made up of a hydrodemetallization catalyst and a hydrotreating catalyst, with extended times on stream and production of high commercial value waxes of high quality.
However, the cited literature does not describe nor suggests the system of mixed catalytic bed now presented as applied to the iso-paraffins-enriched feed of the present invention, the inventive bed being designed to yield lube base oils of high viscosity index as well as diesel oil of high cetane number.
The raw materials useful in the process of the present invention comprise streams from the processes employed for the production of lube base oils and macro- and microcrystalline waxes, the macro-crystalline waxes being generated from light neutral distillate (LN), medium neutral (MN) and heavy neutral (HN), while the microwaxes are generated from bright stock deasphalted oil (BS).
Basically, these processes comprise the vacuum distillation of atmospheric residues which yield several distillate streams (neutral oils) as well as solvent deasphalting of the vacuum residue yielding deasphalted oils which thereafter can be directed to an usual step of aromatic removal with solvents to yield raffinate streams which are then submitted to usual solvent dewaxing processes whereby are obtained dewaxed oils and slack waxes.
Sometimes neutral and deasphalted oils are first directed to usual processes for solvent dewaxing and the dewaxed oils are then submitted to usual processes of solvent aromatic extraction.
The slack wax which is a by-product of the solvent dewaxing processes is so called in view of the fact that it contains a certain amount of lube base oil occluded in the filter cake during the filtration step of the dewaxing processes (as can be seen through ASTM Methods no D-721 or D-3235). The oil occluded in this paraffin contains most of the aromatic and heteroatom compounds present in the slack wax.
Slack waxes can then be submitted to a further deoiling process whereby are separated from this stream the high melting point and high comercial value fractions--the hard waxes--and a stream of low commercial value--the soft waxes, which usually make up the feed of fluid catalytic cracking units (FCC).
The soft wax stream, if deprived of its lube fraction would still show a melting point which would be too low for such uses as candles, packing, etc. however not low enough for applications such as lubes and liquid fuels, which are among the applications sought by the present invention.
The hard waxes yielded by the conventional deoiling process (melting point higher than 46.degree. C.) show a composition wherein 50 to 90% are n-paraffins while 10 to 40% are iso-paraffins. The soft waxes obtained in this same process show in their composition of from 20 to 80% of iso-paraffins the balance to 100% being n-paraffins based on the total paraffins, as illustrated below:
______________________________________ n-Paraffin content i-Paraffin content ______________________________________ Hard wax from heavy neutral % 66 34 Soft wax from heavy neutral % 31 69 Slack wax from heavy neutral % 38 62 ______________________________________
As stated in EP 0464561, the slack wax from the dewaxing process can be separated in an initial stage into harder waxes which would then be the feed of the hydroisomerization process and a stream called "foots oil". In spite of the fact that this publication stresses the need to effect an initial deoiling as a way of removing from the raw material of the hydroisomerization process most of the aromatics and heteroatoms present in the feed (slack wax), this additional step represents additional investment and/or production costs, besides wasting a portion of the normal and iso-paraffins removed in the "foots oil".
Among other raw materials useful in the present invention are the soft macro- and micro crystalline waxes which constitute by-products of the usual process for the deoiling of slack waxes and petrolatums in which the main products are the commercial macro- and/or microcrystalline hard waxes. Most preferred are the soft waxes generated from the slack waxes yielded in conventional solvent dewaxing processes which comprise two or more filtration steps since these waxes show a lower content (zero to 10%) of occluded lube base oil as can be seen by applying ASTM Method D-721. Thus, the raw material called soft wax is made up of occluded lube base oil plus low melting point paraffin.
The choice of the soft waxes as the main raw material of the inventive process shows two advantages as compared to other raw materials: lower intrinsic worth in view of the absence, in this stream, of the higher melting point and high commercial value portion (that is, the hard waxes), thus reducing production costs, besides the fact that, as a natural consequence of its production processes, the soft wax stream is intrinsically enriched in iso-paraffins, this being advantageous in the hydroisomerization step.
However, the presence of lube base oil occluded in the soft wax requires for this feed a proper treatment in a catalytic bed in the presence of hydrogen in order to remove compounds such as asphaltenes, polynuclear aromatics, nitrogen, sulfur and oxygen, which are harmful to properties such as viscosity index, oxidation stability, etc.
Other raw materials adequate for the inventive process comprise slack waxes which originate from the dewaxing of raffinates and of atmospheric or vacuum distillates employed in the manufacturing of lube base oils, deoiled waxes known as hard waxes and which originate from these same distillates, petrolatums obtained from deasphalted residues, microcrystalline hard waxes, gasoils from atmospheric or vacuum distillation, raffinates from conventional processes for aromatic extraction from atmospheric or vacuum distillates and specially the so-called soft waxes which constitute the by-product of the usual deoiling of the macro- and microcrystalline slack waxes and petrolatums.
Therefore, one objective of the present invention is lube oils of high viscosity index and pour point -6.degree. C. or less from iso-paraffins enriched feeds such as light neutral slack waxes LN, medium neutral MN, heavy neutral HN and bright stock BS, or soft waxes LN, MN and HN as well as microparaffins.
Another objective is a catalytic bed wherein the feed contacts successively a hydrodemetallization bed, a hydrotreating bed, a hydroisomerization/hydrocracking bed and optionally a hydrofinishing bed, whereby lube oils of outstanding characteristics are produced.
Still an objective is to obtain diesel oil of high cetane number, in the distillation range between 150 to 370.degree. C., from the same feeds and catalytic bed used to make lube oil, by recycling to the process feed the 370.degree. C.+ fraction.
An additional objective is to provide a process for producing lube base oils and diesel oil of high quality from paraffinic feeds of low cost and commercial catalysts, this representing great process savings.